1. Field of the Invention
The invention relates to a method of correcting the phase of a complex magnetic resonance spectrum obtained, using Fourier transformation, from sampling values of at least one resonance signal, which resonance signals are generated by means of RF electromagnetic pulses in an object which is situated in a steady, uniform magnetic field, a model of the complex magnetic resonance spectrum being formed by means of the magnetic resonance spectrum obtained, the phase being corrected on the base of the model.
The invention also relates to a device for determining a complex magnetic resonance spectrum of at least a part of an object, which device comprises means for generating a steady magnetic field, means for applying magnetic field gradients on the steady, uniform magnetic field, means for transmitting RF electromagnetic pulses in order to generate resonance signals in the object, means for receiving and detecting the resonance signals generated, and means for generating sampling values from the detected resonance signals, and also comprises programmed means for determining, using Fourier transformation, the complex magnetic resonance spectrum from the sampling values, which programmed means are also suitable for determining a model from the complex magnetic resonance spectrum.
2. Description of the Prior Art
A method of the kind is known from the article by J. Daubenfeld et al. in "Journal of Magnetic Resonance", Vol. 62, pp. 195-208, 1985. Said article describes an automatic phase correction for a complex magnetic resonance spectrum containing phase errors. The phase errors are caused inter alia by misadjustment of the detector and by instrumental deficiencies such as the use of non-ideal analog anti-aliasing filters and the waiting period required, after the application of the RF electromagnetic pulse, before the signal sampling of the magnetic resonance signal (FID signal) commences. In the complex spectrum obtained from the sampling values the phase errors occur as frequency-independent (zero-order) and frequency-dependent (higher-order) phase errors. The cited article proposes a method of recovering an absorption component from the complex magnetic resonance spectrum containing phase errors by means of automatic phase correction. A calibration curve for the phase is determined from phases of preferably three reference lines. When the phases of the reference lines are known, in the case of three reference lines a second-order phase polynomial (the calibration curve) is adapted thereto by means of a least-squares criterion. The coefficients of the phase polynomial are then defined, so that the spectrum can subsequently be point-wise corrected. The absorption component can be determined from the corrected spectrum (see formule 6 on page 199 of the cited article). The phases of the reference lines are determined on the basis of a parametric model of the waveform of the signal. This model contains a number of parameters of the resonance line, such as signal amplitude, resonance frequency, line width at half amplitude, and differential angle between the actual (unknown) phase and an approximative phase of the (unknown) actual phase. Using a least-squares optimization procedure, the model is adapted to (measured) data points of the complex magnetic resonance spectrum. The approximative phase is varied until, for example a minimum differential angle is reached. The approximative phase is then taken as the phase for the reference line. The described procedure is repeated for each reference line. The method described in the cited article has the drawback that the model parameters must be accurately determined. If the spectrum does not contain well-defined resonance lines which can serve as reference lines (in the case of non-defined resonance lines the model parameters cannot be determined with adequate accuracy), it will be necessary to add substances to the object which can serve as the reference in that they cause an acute resonance peak in the spectrum. In said article, for example an oil spectrum is measured whereto the reference substances TS CCL.sub.4 and CS.sub.2 have been added, which additions produce well-defined resonance lines in parts of the spectrum which are not of importance.
It is the object of the invention to provide a method which does not have the said drawbacks.